The ability of the developing, non-precocial newborn mammal to produce mature, biochemical, physiological, and behavioral responses following drug treatment may depend upon the functional status of monoamine-containing neurons. Such monoaminergic neurons are not fully mature at birth in neonatal animals: They are less able to synthesize, store, inactivate, release, and respond to neurotransmitters than are corresponding neurons in adult animals. Previously observed changes in the ontogenesis of activity, food consumption, and thermoregulation in the neonatal rat may reflect the gradual maturation of neurochemical mechanisms that mediate these responses. Alterations in these responses produced by drugs thought to act via monoaminergic neurons also change as a function of the age of the organism. The research is aimed at: 1) determining whether the spontaneous developmental changes in activity, feeding, and thermoregulation can be correlated with the biochemical maturation of brain and/or peripheral monoaminergic neurons; 2) determining whether similar correlations exist between the developmental physiological and behavioral effects of drugs such as amphetamine that might exert their action on specific sets of monoamine-containing neurons; and 3) examining some of the mechanisms that may underlie the developmental changes in normal and drug-induced responses.